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Low Mass Standard Model Higgs Boson Searches at the Tevatron

Low Mass Standard Model Higgs Boson Searches at the Tevatron. Andrew Mehta. On behalf of the CDF and D0 Collaborations. Physics at LHC, Split, Croatia, September 29th 2008. Tevatron. Tevatron is running really well :. Lumi per experiment.

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Low Mass Standard Model Higgs Boson Searches at the Tevatron

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  1. Low Mass Standard Model Higgs Boson Searches at the Tevatron Andrew Mehta On behalf of the CDF and D0 Collaborations Physics at LHC, Split, Croatia, September 29th 2008

  2. Tevatron • Tevatron is running really well: Lumi per experiment Analyses in this talk use 0.9 - 2.7 fb-1 per experiment Expect 6-8 fb-1 datasets by end of 2009 + possibly run in 2010 Data analysed with 2 multipurpose detectors CDF and D0: EM +had calorimeters for e +jet id Muon detectors Scintillator fiber/drift chamber Silicon vertex detectors for b tagging Andrew Mehta

  3. Higgs Production at the Tevatron and Decay Tevatron s=1.96 TeV Higgs Production Cross Section [pb] Higgs Branching Ratio Hxx Good at high mH see next talk ttH Additional low mass search channels: Main channels at low mH<130 GeV WH  vbb VH  qqbb H   (with jets) H   ttH  lvbbbbqq WH  lvbb ZH  llbb, vvbb Dominant gg H, H  bb has massive background Andrew Mehta

  4. The Challenge • Higgs production is very rare. • Careful analysis to separate signal • from background. • Trigger • High pT e, triggers • MET + Jets triggers • Track + MET + Ecal -trigger • Reconstruct final state • Leptons • Efficient b-tagging • Good jet resolution • MET reconstruction • Separate signal and background • Measure backgrounds • Advanced analysis tools to separate signal from background (NN, ME, BDT) 1 0.03-0.3 Higgs ggH Andrew Mehta

  5. NN Inputs: NN Inputs: WH  l  b b b Analysis Technique • Loose double or tight single b tagging • Lepton ID enhanced with isolated tracks, forward e + extended muons • Include W+3 jet data • NN discriminator • ME+BDT H b W* W l  • 1 lepton+MET+ 2 b jets final state • About 3-4 evts / 1fb-1 • Most sensitive channel at low mass at present Main background W+bb 115 GeV (X10) Andrew Mehta

  6. Analysis Lum (fb-1)‏ Higgs Events Exp. Limit Obs. Limit CDF NN 2.7 8.3 5.8 5.0 CDF ME+BDT 2.7 7.8 5.6 5.7 DØ NN 1.7 7.5 8.5 9.3 WH  l  b b Neural Network Matrix Element+ Boosted Decision Tree signal background background signal Results at mH =115GeV: 95%CL Limits/SM Andrew Mehta

  7. ZH    b b b Analysis Technique H Z* b • Use data to estimate multijet background • Add 3rd jet to get acceptance for WHbb (CDF) • or dedicated analysis (D0) • CDF adds tracks to jets to improve resolution (H1 alg) • D0: BDT with 24 inputs • CDF: Uses NNs Z   • Large MET + 2 b jets • Also add WH(l)bb when lepton is missed • ~ 3-4 evts / fb-1 Dijet Mass MH=115GeV Main backgrounds Z+bb, W+bb, multijets

  8. Analysis Lum (fb-1)‏ Higgs Events Exp. Limit Obs. Limit CDF NN 2.1 7.3 6.3 7.9 DØ BDT 2.1 3.7 8.4 7.5 ZH    b b BDT Analysis NN Results at mH =115GeV: 95%CL Limits/SM

  9. ZH  l l b b b Analysis H Z* b • Improved dijet mass resolution with no MET constraint • Loose double/tight single b tagging • CAL/track only leptons • NN, BDT, ME techniques Z l l • 2 leptons + 2 b jets  Cleanest signature! No MET, can fully reconstruct event • About 1 event / 1fb-1 Main background Z+bb Improvement in massresolution MH=120GeV/c2 After constraint Standard jet corrections Andrew Mehta

  10. Analysis Lum (fb-1)‏ Higgs Events Exp. Limit Obs. Limit CDF NN 2.4 1.8 11.8 11.6 CDF ME(120)‏ 2.0 1.4 15.2 11.8 DØ NN,BDT 2.3 2.0 12.3 11.0 ZH  l l b b Boosted decision tree Matrix Element Results at mH =115GeV: 95%CL Limits/SM Limit as good per fb-1 as WH channel CDF Run 229879 Event 3787664 Andrew Mehta

  11. Other channels sensitive at low mass Dijet Mass Analysis Technique • Signature had+lep+2 jets • Simultaneous search in WH+ZH+VBF+ggH • Use NN to extract signal • Good proof of principle for LHC b H W* b W   Analysis Technique • Hadronic  + MET + 2 b jets • Use Dijet mass to extract signal Andrew Mehta

  12. Other channels sensitive at low mass b H Analysis Techniques W*/Z* b • 1 lepton + MET + 4 or 5 jets • Separate events into • 1,2,3, 3 b tags • Use scalar sum of jets (HT) to extract signal W/Z q q 1.3 signal events • 4 jets, ≥2 b jets • Large BR of W/Z→qq • Large multijet background • Use matrix element to extract • signal H    • 2 photons • NN photon id • Low BR H   • Background QCD   ,  jet • Use di-photon mass to extract signal Andrew Mehta

  13. Summary of Analyses mH=115 GeV/c2 (mH=120 GeV) Also WW contributes in the low mass region Andrew Mehta

  14. Tevatron Combination - Status of April 9th 2008 - Updated Tevatron combination in progress! Andrew Mehta

  15. Conclusions • Higgs physics at the Tevatron is getting exciting! • Low mass region has large backgrounds, but can be suppressed by multi-variant techniques and understood in control regions • Expected limit should fall below ~ 3 x SM for mH=115 GeV/c2 in the next Tevatron combination • Additional improvements actively in progress • Further extending signal acceptance for leptons and b tagging • Improved jet resolution • Extended b-tagging and flavour separators • Expect 2-3 times current analyzed lumi (more if we run in 2010) • Details on each analysis is available at: • CDF: http://www-cdf.fnal.gov/physics/new/hdg/hdg.html • D0: http://www-d0.fnal.gov/Run2Physics/WWW/results/higgs.htm Andrew Mehta

  16. Indirect limits Supersymmetric Higgs is also low mass If Higgs is Standard Model Higgs, mass is likely to be low Andrew Mehta

  17. Identification of b-quarks (b-tagging) • Most sensitive channels have Hbb • Silicon detectors used to find secondary vertices • Efficiency ~40 - 70% • Fake rate (mistags) typically 0.5 - 5% • D0 uses Neural Network tagger based on b-lifetime information. Can use multiple operating points. • CDF utilizes secondary vertex and Jet Probability algorithms + additional NN flavor separator • Use either single tag or looser double tag D0 c/l-like b-like CDF CDF Jet-Flavor Separator “b-tag” = Identified 2nd vertex Neural Network Output Andrew Mehta

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